The present invention is directed to a process and apparatus for straightening weft yarns in fabrics.
Such process is known generally from Textile Practice International, Oct. 1986, pages 1115-1116. During the production of a normal fabric in a weaving machine, the wrap and weft yarns intersect precisely at right angles. However during the different working cycles in the equipment, the fabric can often become distorted. This distortion must be compensated or eliminated for different reasons.
Straightening devices of different kinds are available for correcting weft yarn distortions. The essential question here concerns roller assemblies disposed diagonal to each other. In addition, there are known differentially operating straightening machines wherein both chain drives of a tentering frame are controlled differently so as to align the weft yarns perpendicular to the direction of advance. But in all these straightening machines, it is necessary, in the first place, to determine the course of the weft yarns to be able then to carry out an adequate motor-driven adjustment of the straightening elements.
An essential advantage of the alignment when tension is simultaneously applied in the direction of the weft yarn is that the S-shaped and wavy distortions, etc. are automatically compensated to a great extent due to the stretch of the fabric.
It has been known for many years that an "automatic" compensation of the distortion can be obtained by needling the edges of the fabric web on wheels which have their axes of rotation disposed diagonally to the direction of movement in a manner such that the fabric web is substantially needled without width elongation and is then stretched during the (partial) rotation. The wheels here are free-wheelingly fastened upon their shafts. As long as the weft yarns are perpendicular to the direction of movement, that is, without distortion, the forces acting upon both wheels are equal during the tension. But as soon as a diagonal distortion is present in the fabric, a force acts between the wheels in the longitudinal direction of the fabric that brakes the wheel to the side with the "current" weft yarns and accelerates the wheel to the other side (lagging weft yarn). Among others, an essential problem here consists in that the needling on the wheels is really different and often results in the fabric being torn or moving down from the wheel. In published European Patent Application No. 136 115, there is described an assembly in which this disadvantage can be prevented. But even in this assembly, the needling is relatively difficult. Moreover, an additional problem appears in this assembly (as also in the former assemblies). Such problem directly results from the fact that the clamping wheels move freely and the fabric is removed by a take-up roller so that a curved distortion occurs since the fabric has been "braked" on its edges.